A command or control signal consists of a particular item of information produced by a device for producing control signals, for example a switching device or a potentiometer, which is actuated for example either by a suitable automatic control circuit or manually by an operator.
In a first group of arrangements in the prior art, it is known to produce items of information, for example in binary form, comprising a succession of "0" and "1" bits over a period of time. Such an item of information is represented by a succession of such bits in accordance with a predetermined syntax.
Such apparatus, if it is arranged to be connected to a device for producing control systems for controlling a plurality of loads as in the case of a vehicle, calls for time-based multiplexing, in particular for the purpose of reducing the number of lines constituting the transmission channel. This reduction in the number of lines has become an important goal, due to the cost involved in providing the cumulative length of the wires necessary in conventional arrangements, and also because of the difficulties of forming suitable cable harnesses.
Such a multiplexing device firstly involves the use of a communication protocol which must be created with the aid of specialised circuits. Secondly, it calls for a large number of signal transmission lines. This increases the cost of the transmission function within the vehicle, since the loads are usually located remotely from the control means from which the items of information originate, or remote from the sensors which are required for the purpose of activating the loads or the control means, which may for example be on the fascia of the vehicle.
Finally multiplexing calls for means which are perfectly adapted to the environment of the vehicle, in particular by being insensitive to external perturbations while remaining able to operate even with comparatively large variations in voltage. In this connection, the basic supply voltage is usually provided by the battery of the vehicle, and as is well known, the nominal voltage output of the battery is caused to vary quite considerably, depending on the load imposed on the battery.
These various elements require the multiplexing devices for a vehicle to be relatively costly, and they are characterised by a number of disadvantages.
In another type of arrangement in the prior art, it is known to produce items of information while reducing the number of transmission channels. In those arrangements this is achieved by transmitting the items of information, not in binary form as in the systems mentioned above, but in multi-value forms, that is to say forms in which each unit of information passing along a transmission channel assumes a state which is selected from a plurality of states, which represents an elementary word in a syntax for producing the totality of items of information which are required on the transmission channel.
Such an apparatus requires very precise generation, over the appropriate period of time, of the various events that enable the items of information to be generated. Such an arrangement is again comparatively costly, and is in fact not well adapted to automotive use.